Production of long chain hydrocarbon molecules through Fisher Tropsch (hereinafter also referred to as “FT”) reaction is well known for more than fifty years and commercially being practiced in quite a few places. Recently there has been renewed interest due to volatility in crude prices and consequent emphasis on more profitable use of huge coal and gas reserves. Iron or cobalt catalysts are typically used for converting CO, H2 rich synthesis gas into liquid fuels in fixed or slurry bubble column reactors. Much of the early work has focused on fixed bed reactor systems. However, recent trend has been to use slurry bubble column reactors due to relative ease of handling huge exothermic heat of reaction from FT reactions.
Slurry bubble column reactors (SBCR) operate with catalyst particles suspended in liquid phase, while the synthesis gas is sparged at high pressure from the bottom of the reactor. Due to concentration driving force, the reactor liquid absorbs the gas from the rising gas bubbles and FT reactions occur over the suspended catalyst particles producing both gas and liquid hydrocarbons. Depending on the partial pressure of the hydrocarbons the product molecules remain either in liquid or gas phase. The key advantages of SBCR system includes excellent heat transfer performance, online catalyst addition and removal, and reasonable inter phase mass transfer rates with low energy input.
Several studies focused on improving the SBCR designs for improving the reactor productivity and selectivity. In U.S. Pat. No. 7,019,038, a method for reducing the molecular Weight of liquid hydrocarbon through recycling a portion of lower molecular weight hydrocarbon product was disclosed. It was expected that by lowering the molecular catalyst site, thus improving the conversions. Similarly, U.S. Pat. No. 6,897,246 suggests the locations for recycling olefins streams in a multi stage reactor system for improved light olefin conversion. U.S. Pat. No. 5,827,902, suggests a FT reactor configuration with more than two slurry reactors in series or placing baffles inside the reactor for achieving plug flow contacting pattern and improving reactor productivity. The benefits envisaged through this system was higher overall feed conversions and less recycle requirement.
Although multi stage configurations aid in improving the reactant conversion, the conversion in each stage is different and hence requires different heat transfer area for absorbing the heat. This results in major differences in reactor sizes due to non-uniform heat exchanger sizing. Uniform size reactors are preferred from maintenance and cost perspective. Similarly, Olefins produced during the course of reaction would exit the system, unless separate arrangements are made through recycling. It is expected that the recycling of light olefins would undergo further chain growth leading to more favorable middle distillates and liquid fuels production.
Hence, it is desirable to have reactor design configurations that are more uniform and simultaneously enhances the productivity and selectivity. It is believed that by devising the gas liquid contacting pattern, the reactor productivity and selectivity are greatly improved. This scheme also result's in much narrower product distribution.